The normal preception of sound occurs in a person when sound waves strike the tympanic membrane and cause it to vibrate. These vibrations are transmitted through the tiny bones of the ossicular chain in the middle-ear to the cochlea in the inner ear, which results in electrical impulses being transmitted through the auditory or cochlea nerve to the brain. Even if the sound conducting mechanisms of the middle-ear are functioning perfectly, a hearing loss can be experienced if the inner ear is damaged.
The more conventional hearing aid devices are of the "air conduction" type and are used to overcome a hearing loss due to inner ear damage (sensorineural loss) and/or hearing loss due to a mild impediment of the sound conducting mechanisms of the middle-ear. Such air conduction hearing aid devices work by simply amplifying the incoming sound and delivering the amplified sound signal by way of a speaker positioned in the outer ear canal. This amplified sound simply "overdrives" the ear's sound conducting mechanism. Since an air conduction hearing aid device must have some of its componentry in the outer ear canal, and since it also requires a fairly normal tympanic membrane and ossicular chain in the middle-ear, some hearing impaired persons are unable to derive any benefit from the air conduction hearing aid.
Persons who cannot benefit from an air conduction hearing aid can sometimes benefit from a "bone conduction" hearing aid. A bone conduction hearing aid works by converting the sound signal into a mechanical vibratory stimulus. Heretofore, the vibrating portion of most commercially-available hearing aids of this type has been placed against the skin, usually behind the ear, under some pressure. The vibrator transmits its vibrations through the skin and soft tissue into the bone structure of the skull. The vibration of the skull stimulates the cochlea and a sound is perceived. Such commercially available bone conduction hearing aid devices are not very popular due to several limitations. First, the devices are bulky and must be worn on a headband or a special eyeglass frame in order to keep the vibrator pressed tightly against the skull. In addition, because the vibration must be transmitted through the skin and soft tissue overlying the skull, the fidelity of sound and the efficiency of these types of hearing aid devices are poor.
Proposals have been made for improving bone conduction devices for stimulating the inner ear. One such proposal is disclosed in U.S. Pat. No. 3,290,081 in which a radio receiver is implanted underneath the skin and includes a vibration generating means which is connected to the temporal bone subcutaneously. A transmitter may be located at any remote place on the body of the user within the range of the implanted radio receiver for generating a modulated signal in response to sound received by a microphone. This modulated signal is received by the radio receiver and the vibrator is caused to vibrate in response to the modulated signal and set up vibrations within the temporal bone which in turn stimulates the inner ear to create a preception of sound. This implanted radio receiver is quite complex and includes numerous implanted electronic components including a power supply, which are susceptible to malfunction and other potential problems which could cause extreme difficulty due to the implanted nature thereof.
Another proposal relates to some experimental work conducted in Europe and described in a recent published paper wherein a direct bone conduction hearing aid device was implanted which included a bone screw implanted directly in the temporal bone subcutaneously and a post connected directly thereto. This post extends percutaneously (through the skin) to a location externally of the skin. A vibrator which creates vibrations in response to a modulated signal is connected to this post and vibrations are transmitted by the post to the bone screw and thence to the temporal bone of the skull to stimulate the inner ear and create the preception of sound. This device has distinct disadvantages, not the least of which are the likelihood of infection and the undesirability of a ceramic element extending permanently through the skin from aesthetic, psychological and comfort standpoints.
To overcome the problems presented with the above previously-proposed bone conduction hearing aid devices, a development was made, as disclosed in co-pending U.S. application Ser. No. 674,176, filed Nov. 23, 1984, and assigned to the assignee of the present application, in which a direct bone conduction hearing aid device is proposed in which the signal transmitting device is held in place without unsightly or uncomfortable external devices. This bone conduction hearing aid device includes sound processing mechanism for converting sound into an analog electromagnetic signal and having an output transmitter for transmitting the electromagnetic signal and being adapted to be placed supercutaneously on the skull of a hearing impaired person and having a first magnetic means therein. The device further includes a vibration generating mechanism adapted to be implanted subcutaneously and secured to a skull bone of the hearing impaired person and second magnetic means. This second magnetic means cooperates with the first magnetic means to hold said transmitter in position supercutaneously on the skull of the hearing impaired person, (2) receives the electromagnetic signal from said transmitter of said sound processing means, and (3) vibrates the skull bone in response to such electromagnetic signal. With this device, vibrations are generated subcutaneously in response to the analog electromagnetic signal and conducted through the bones of the skull to stimulate the inner ear to create the perception of sound in the hearing impaired person.
While this development of the aforesaid co-pending patent application performed satisfactorily for many cases, there are situations in which direct vibrations of the ossicles of the middle-ear is more desirable and may provide a more enhanced perception of sound in the hearing impaired person.
For these latter described cases, there has been proposed in U.S. Pat. No. 3,870,832 an implantable middle-ear bone-conduction hearing aid device which utilizes a sound transducer means for converting audio signals to electric signals and electromagnetic transducer means for receiving the electrical signals and converting the electrical signals into mechanical movement of the stapes bone of the ossicular chain in the inner ear. The device of this patent includes a magnet that is permanently attached to the stapes of the ossicular chain in the inner ear, an electromagnetic coil implanted in the temporal bone of the user and extending through the skin to be attached to a sound processing means outside the skin. The electromagnetic transducer receives electrical signals and transmits them by way of electrodes extending therefrom and into the middle-ear of the user and attached to the magnet secured to the stapes of the ossicular chain in the middle-ear of the user.
While this device of U.S. Pat. No. 3,870,832 directly stimulates the ossicular chain of the inner ear of the user by bone conduction, it has many disadvantages and limitations including the following. The hearing aid device requires a percutaneous element extending through the soft tissue and skin of the user behind the ear which creates the likelihood of infection from permanently open skin and bacterial invasion. It is also unacceptable from cosmetic, psychological and comfort standpoints. Secondly, this patented device utilizes electrodes which extend directly into the middle-ear of the user and are very difficult to implant surgically and open up strong possibilities of electrode fractures, insulation leaks and current leaks. Thirdly, this device only provides for a vibration generating mechanism to be attached to the head of the stapes of the ossicular chain in the middle-ear. In many cases, the promontory is too high on the inferior side to allow attachment of a vibration generating mechanism to the head of the stapes. Also, there are occasions when the facial ridge impinges on the stapes and would make such application impractical. In summary, there are many cases in which attachment of a vibration generating means to the head of the stapes in the ossicular chain will not provide the desired stimulas to the inner ear to create the required preception of sound in the hearing impaired person.